1. Field of the Invention
The invention relates to an image reading apparatus for reading, at a reading position, an image on a sheet of a document fed from a sheet supply tray to a catch tray along a feed pathway.
2. Description of Related Art
There is known an image reading apparatus which is incorporated in a copy machine, a scanner, and a multifunction apparatus implemented with a copy function and a scanner function, and which comprises a document feeding mechanism for feeding a sheet of a document from a sheet supply tray to a catch tray through a feed pathway, and an image reader such as CCD (Charge Coupled Device) and CIS (Contact Image Sensor), disposed at a position in the document feeding mechanism to read an image on the document sheet as being fed through the feed pathway.
In a conventional image reading apparatus, an automatic document feeding mechanism called ADF (an Auto Document Feeder) feeds a sheet of a document onto a platen glass, and an image scanner under the platen glass reads an image on the document sheet as being fed over the platen glass. A guide device consisting of a plurality of guide parts are disposed to be opposed to the platen glass so as to guide the document sheet to a predetermined position on the platen glass. The ADF includes a feeder roller, and a plurality of pinch rollers arranged along a circumference of the feeder roller, and the document sheet is fed along a feed pathway by being nipped between the feeder roller and each of the pinch rollers.
The CCD or CIS used as the image sensor has a light source which emits light toward the platen glass. The light emitted from the light source is reflected by a surface of the document sheet being fed over the platen glass, and incident back on the image sensor. The image sensor converts the received light into an electrical signal. When reading the image in this way, a problem is encountered that where the document sheet is relatively thin, an image on a backside of the document sheet which is opposite to a side whereon the image to be read is present undesirably shows through the thickness of the document sheet, as described in JP-A-2002-84424 and JP-A-2003-198810.
More specifically, the guide device and the feeder roller are disposed to be opposed to the platen glass. The feeder roller is driven or rotated, and each guide part of the guide device and the feeder roller 94 are spaced from each other by a suitable amount of clearance. Thus, at the position to be opposed to the platen glass, each guide part of the guide device and the feeder roller are disposed with the clearance therebetween in a direction of a width of the document sheet, which is parallel to a direction of a driving shaft of the feeder roller.
When the thin document sheet is fed over the platen glass, the light emitted from the image sensor is partially transmitted through the document sheet and reaches the guide device and the feeder roller to be reflected thereby. The thus reflected light is again transmitted through the document sheet to be incident back on the image sensor. The part of the light reaching the clearances between the feeder roller and the respective guide parts of the guide device, travels straight into the ADF, and accordingly does not return to the image sensor. In this way, at places where the guide parts of the guide device and the feeder roller are present, the light as reflected by the guide device and the feeder roller, and the light as reflected by the document sheet, are received in superimposition by the image sensor, while at places corresponding to the clearances between the feeder roller and the respective guide parts of the guide device, only the light as reflected by the document sheet is received by the image sensor. Since reflectance is different between the guide device and the feeder roller, and the emitted light is not reflected at the clearances between the feeder roller and the respective guide parts of the guide device, the image as read by the image sensor includes a show-through in the form of stripes in a direction of feeding of the document sheet and corresponding to the variation in intensity of reflected light, including the non-presence of reflected light at the places corresponding to the clearances.
In a technique disclosed in the above-mentioned first publication JP-A-2002-84424, a plurality of document sheets having respective levels of light transmittance are read in order to obtain data representative of a relationship between light transmittance of document sheet and show-through, in advance of an actual use of the image reading apparatus. The obtained show-through-transmittance relationship data is stored in a storage medium for an apparatus incorporating the image reading apparatus. When images on sheets of a document are to be read, the transmittance of each document sheet is initially measured. Then, show-through information corresponding to the measured transmittance value is retrieved from the stored show-through-transmittance relationship data. Then, image data representative of an image on one of the document sheets is obtained by reading that image on the document sheet, and the image data is corrected based on the show-through information by eliminating a show-through component from the image data,
In a technique disclosed in the above-mentioned second publication JP-A-2003-198810, when an image of a document is to be read, the transmittance of a sheet of the document is measured first. Then, relevant control variables, such as amount of light to be emitted from the image sensor toward document sheet, are changed in accordance with the obtained transmittance value. Thus, when the measured transmittance value indicates that show-through will occur, it is controlled such that an amount of light to be transmitted through the document sheet decreases, so as to prevent a show-through component from being included in a read image.
However, according to the technique of the first publication, it is required to measure the transmittance of each document sheet before reading an image, as well as provide calculating means for carrying out operations associated with the image correction. Further, since such operations are carried out while the document sheet is being fed over the platen glass, the rate of reading undesirably lowers. Still further, the measurement for obtaining the show-through-transmittance relationship data is necessitated, and the means for storing this data is also required. The need for the calculating means and the storing means increases the number of components of the image reading apparatus, resulting in an increase in the size and cost of the image reading apparatus.
On the other hand, according to the technique of the second publication, it is required to measure the light transmittance of the document sheet, and change the control variables such as the amount of light emitted by the image sensor toward the document sheet, in accordance with the measured transmittance value. Hence, the reading rate lowers and the cost of the image reading apparatus increases.
Meanwhile, there are known two types of ADF, namely, a downward feeding type as disclosed in JP-A-2003-76074, and an upward feeding type as disclosed in JP-A-2002-335372. In the former type, a substantially U-shaped feed pathway is formed for feeding each document sheet from a sheet feed portion disposed in an upper portion of the ADF, to a catch portion disposed in a lower portion of the ADF, via a reading position. In the latter type, a substantially U-shaped feed pathway is formed for feeding each document sheet from a sheet feed portion disposed in a lower portion of the ADF, to a catch portion disposed in an upper potion of the ADF, via a reading position. In either of the two types, the feed pathway is substantially U-shaped to reverse a document sheet while the document sheet is fed therealong, thereby reducing the overall size of the ADF.
The ADF includes a plurality of feeder roller pairs disposed along the feed pathway so as to feed the document sheet with high accuracy and precision. Each feeder roller pair comprises a drive roller driven by a drive source, and a driven roller disposed adjacent to the drive roller to be driven by the drive roller, so that the document sheet is fed by being sequentially nipped between the roller pairs, along the substantially U-shaped pathway reversing the document sheet.
However, the ADF having the substantially U-shaped pathway with the plurality of feeder roller pairs suffers from a high component cost when the number of the feeder roller pairs is large. Further, because of a variation in diameter of the drive rollers, the rotation varies among the drive rollers, adversely affecting the feeding of the document sheet.